The present invention relates to a process in the fabrication of apertured shadow masks which are used in color television picture tubes. An apertured shadow mask is the thin metal member, having a plurality of apertures therethrough, for aligning the electron beam with the phosphor on the picture tube screen. The apertures are provided in the thin sheet of metal by a photodelineation and etching process, and the apertured metal portion is termed a flat. The metal flat must then be annealed and work hardened to give sufficient structural strength to be formed into the shadow mask usable in a picture tube. The annealing process is carried out in a reducing atmosphere, typically at about 1800.degree. F for about 45 minutes, as is well known in the art. The typical shadow mask flat is about 6 mils in thickness and has an area which depends upon the size of the picture tube to be produced, but is typically a rectangular body several feet by several feet. The annealing process is typically carried out using an annealing conveyor upon which the shadow mask flats are disposed in passing slowly through the high temperature furnace reducing atmosphere. Because of the tremendous number of pieces which must be processed and the relative thinness of the shadow mask flats, it is desirable to stack the shadow mask flats and to pass the stacked unit through the annealing furnace. It is very difficult to separate the individual shadow mask flats from the annealed stack of flats. It has been the practice to effect such separation manually and by just physically forcing or prying the members apart at their edges. The stack of shadow mask flats comes out of the annealing furnace as a relatively rigid unit which is very difficult to separate and which must be separated without damage or deformation of the individual thin flats.
It is the practice in the annealing stage to place the stacked shadow mask flats between an upper and lower carrier plate which carries the stack of flats on the annealing conveyor through the furnace. Obviously, the higher number of individual flats which can be stacked for passage through the annealing furnace, the greater the manufacturing efficiency. A particular problem has been encountered in separating stacked apertured shadow mask flats of the slot mask type. A slot aperture shadow mask is one in which the apertures are generally rectangular rather than the conventional circular aperture mask. The slotted masks tend to be much more difficult to separate upon completion of the annealing of a stack of slotted aperture flats.
It has been the practice in the handling of sheet metal stacks to separate stuck together sheets by first bending the sheets and then rolling the bent sheets flat, as seen in U.S. Pat. No. 558,457. U.S. Pat. No. 712,617 teaches the separation of metal sheets from a stack in which the sheets are stuck together, by bending the stack alternately about upper and lower rollers. The rollers of this early teaching are alternate with the lowermost extending perimeter portion of the upper rollers projecting below the uppermost extending perimeter of the lower rollers. The stack of sheets must be bent about each succeeding roller as the stack advances. The teaching is directed specifically to tin sheet which is typically relatively thick and flexible.
The shadow mask flats are very thin sheets of metal, typically six mils thick. The apertured flat is particularly susceptible to permanent deformation because of these apertures.